A prior-art travelator diagrammatically presented in FIGS. 1 and 2. This type of travelator comprises a conveyor, which conveys passengers from one end of the travelator to the other. The conveyor comprises a frame. The frame is divided into two parts. The first frame part 3 is essentially fixed in its position, and the second frame part 4 is controlled to move in relation to the first frame part. A drive wheel 5 is mounted onto the first frame part 3 on a bearing allowing rotation, and a power unit 6 is arranged to rotate said drive wheel. A diverting wheel 7 is mounted onto the second frame part 4 on a bearing allowing free rotation. A traction element 9 formed as an endless loop, to which the transport surfaces 8 are connected, moves the transport surfaces 8 along an endless ring-like and flat-shaped path of travel with traction element 9 being led to pass over the drive wheel 5 and the diverting wheel 7. To achieve suitable tightening force and pre-tensioning a tightening device 10 is arranged for the traction element 9, which acts between the first frame part 3 and the second frame part 4 such that it tries to move the diverting wheel 7 linearly away from the drive wheel 5. The traction element 9 comprises an upper section 14, in which the transport surfaces 8 connected to the traction element travel from the first end to the second end of the conveyor 1 in order to convey passengers, and a lower section 15, in which the transport surfaces return from the second end back to the first end.
Tightening is required so that the traction element does not slip off the drive wheel if force is transmitted between the traction element and the drive wheel by means of friction, when the traction element is e.g. a belt and the drive wheel is a belt pulley. Similarly if force is transmitted between the traction element and the drive wheel by shaped transmission, such as when the traction element is a sprocket chain and the drive wheel and diverting wheel are sprocket wheels, the purpose of tightening is to prevent the sprocket chain from jumping off the sprocket wheel.
The conveyor 1 can be driven in the forward drive direction I as shown in FIG. 1, in which the travel direction of the upper section 14 of the traction element is from the diverting wheel 7 towards the drive wheel 5, and in the reverse drive direction II of FIG. 2, in which the travel direction of the upper section 14 is from the drive wheel 5 to the diverting wheel 7.
Although the conveyor would preferably be driven mainly in the forward drive direction of FIG. 1, there may sometimes be a need to drive it in the reverse drive direction according to FIG. 2.
One problem is the minimum tightening force needed by the traction element, which is substantially greater in the reverse drive direction than in the forward drive direction. Furthermore the load exerted on the conveyor affects the tightening force needed.
FIG. 1 presents the force situation in the forward drive direction. In the forward drive direction the minimum tightening force isFtmin=2×Fplt+Fput+Fpl where    Ftmin=minimum tightening force    Fplt=friction caused by the lower section of the conveyor    Fput=friction caused by the upper section of the conveyor    Fpl=friction caused by the load conveyed by the conveyor
In the reverse drive direction of FIG. 2 the minimum tightening force isFtmin=2×Fput+2×Fpl+Fplt where    Ftmin=minimum tightening force    Fplt=friction caused by the lower section of the conveyor    Fput=friction caused by the upper section of the conveyor    Fpl=friction caused by the load conveyed by the conveyor
In the reverse drive direction the tightening of the traction element must overcome the friction force of the whole conveyor and load. Therefore the traction element must be pre-tensioned to a much greater force compared to the forward drive direction.
If the travelator is equipped with a tightening device, which provides a constant force, such as a spring or similar, it would have to be dimensioned to provide the greater tightening force that the reverse drive direction requires.
One problem with this is that the traction element and the drive wheel and the diverting wheel then wear quickly, in which case their service life and servicing interval become short.